The present invention relates generally to a system for precisely controlling the operational characteristics of a relay. More specifically, the present invention relates to a system for providing constant current to a relay despite variations in supply voltage and temperature.
Relays are generally operated under conditions that do not require precision timing. In some applications, however, precise control of the operational characteristics of a relay may be necessary. Important operational characteristics of a relay include the operate voltage, the release voltage, the operate time, and the release time. For a relay having normally open contacts, the operate voltage is the minimum relay coil voltage required to effect closure of the relay contacts following application of such operate voltage. The release voltage is the maximum relay coil voltage causing the relay contacts to remain closed before the contacts open when such voltage is decreased or removed. The operate time is the time elapsed from an application of the relay coil voltage until the contacts close. The release time is the time elapsed from removal of the relay coil voltage until the contacts open.
Operation of an electromagnetic relay is governed by physical properties such as the mass of moving parts, the frictional forces between components, the mechanical advantages of the design and the magnetic forces generated by a relay motor or solenoid which move a moveable mass to close the contacts. The mass of the moving parts, the component frictional forces and the mechanical advantages required to provide the requisite contact forces are generally unchanged by temperature. The magnetic forces generated by the relay motor or solenoid are directly proportional to the number of coil winding turns and the current flowing through those turns. The number of coil turns is fixed but the resistance of the coil winding material, and thus the coil current, varies with temperature according to the temperature coefficient of resistance of the winding material.
The operational characteristics of a relay are highly dependent on the coil current, which varies in accordance with coil resistance. Therefore, variations in temperature can cause substantial changes in the operational characteristics of relays and also present significant challenges in their design.